Injury to vascular endothelium results in the rapid adhesion of platelets to the exposed underlying subendothelium, an important step in the processes of hemostasis and atherogenesis. This platelet adhesion requires the presence of von Willebrand factor, a multimeric plasma protein, which we have demonstrated also to be present within vascular subendothelium. We plan to identify the subendothelial binding vessel digestion studies, by examining the binding of VWF to purified known subendothelial substituents and by immuno-electron microscopic localization techniques. We also wish to determine whether there is a multimeric preferentiality in the binding of VWF to subendothelial constitutents. Elucidation of the binding of VWF to subendothelium would be an important advance in understanding the mechanism of platelet adhesion.